This invention relates to a Pigment with day-light fluorescence and more particularly, but not exclusively to a pigment absorbing blue to green light and emitting fluorescence within the yellow to red spectral region under excitation by daylight or by an artificial light source. Further absorption in other spectral regions is possible, especially in the UV. More specifically, such a pigment can be used as a phosphor for light sources, especially for Light Emitting Diodes (LED) or electrical lamps. The pigment belongs to the class of rareearth activated silicon nitrides.
For Eu2+-doped material normally UV-blue emission is observed (Blasse and Grabmeier: Luminescent Materials, Springer Verlag, Heidelberg, 1994). Several studies show that also emission in the green and yellow part of the visible spectrum is possible (Blasse: Special Cases of divalent lanthanide emission, Eur. J. Solid State Inorg. Chem. 33 (1996), p. 175; Poort, Blokpoel and Blasse: Luminescence of Eu2+ in Barium and Strontium Aluminate and Gallate, Chem. Mater. 7 (1995), p. 1547; Poort, Meijnhoudt, van der Kuip, and Blasse: Luminescence of Eu2+ in Silicate host lattices with Alkaline earth ions in a row, J. Alloys and Comp. 241 (1996), p. 75). Hitherto, red Eu2+ luminescence is observed only in some exceptional cases, such as in alkaline earth sulphides and related lattices of the rock-salt type (Nakao, Luminescence centers of MgS, CaS and CaSe Phosphors Activated with Eu2+ Ion, J. Phys. Soc. Jpn. 48(1980), p. 534), in alkaline earth thiogallates (Davolos, Garcia, Fouassier, and Hagenmuller, Luminescence of Eu2+ in Strontium and
Barium Thiogallates, J. Solid. State Chem. 83 (1989), p. 316) and in some borates (Diaz and Keszler; Red, Green, and Blue Eu2+ luminescence in solid state Borates: a structure-property relationship, Mater. Res. Bull. 31 (1996), p. 147). Eu2+ luminescence in alkaline-earth silicon nitrides has hitherto only been reported for MgSiN2:Eu (Gaido, Dubrovskii, and Zykov: Photoluminescence of MgSiN2 Activated by Europium, lzv. Akad. Nauk SSSR, Neorg. Mater. 10 (1974), p. 564; Dubrovskii, Zykov and Chernovets: Luminescence of rare earth Activated MgSiN2, lzv. Akad. Nauk SSSR, Neorg. Mater. 17 (1981), p. 1421) and Mg1-xZnxSiN2:Eu (Lim, Lee, Chang: Photoluminescence Characterization of Mg1-xZnxSiN2:Tb for Thin Film Electroluminescent Devices Application, Inorganic and Organic Electroluminescence, Berlin, Wissenschaft und Technik Verlag, (1996), p. 363). For both Eu2+ luminescence in the green and green/blue part of the spectrum was found.
New host lattices of the nitridosilicate type are based on a three dimensional network of cross-linked SiN4 tetrahedra in which alkaline earth ions (M=Ca, Sr and Ba) are incorporated. Such lattices are for example Ca2Si5N8 (Schlieper and Schlick: Nitridosilicate 1, Hochtemperatursynthese und Kristallstruktur von Ca2Si5N8, Z. anorg. alig. Chem. 621, (1995), p. 1037), Sr2Si5N8 and Ba2Si5N8 (Schlieper, Millus and Schlick: Nitridosilicate II, Hoch-temperatursynthesen und Kristallstrukturen von Sr2Si5N8 and Ba2Si5N8, Z. anorg. alig. Chem. 621, (1995), p. 1380), and BaSi7N10 (Huppertz and Schnick: Edge-Sharing SiN4 tetrahedra in the highly condensed Nitridosilicate BaSi7N10, Chem. Eur. J. 3 (1997), p. 249). The lattice types are mentioned in Table 1.
Sulfide based phosphors (e.g. earth alkaline sulfides) are less desirable for lighting applications, especially for LED applications, because they interact with the encapsulating resin system, and partially suffer from hydrolytic attack. Red emitting Eu2+ activated berates show already temperature quenching to a certain degree at the operating temperature of LEDs.
It is, therefore, an object of this invention to obviate the disadvantages of the prior art. It is another object of the invention to provide a pigment for day-light fluorescence. It is a further abject to provide a yellow to red emitting luminescent material which is excitable at wavelengths around 200 to 500 nm, preferably 300 to 500 nm, together with high chemical and thermal stability.
Especially high stability up to at least 100xc2x0 C. is highly desirable for LED applications. Their typical operation temperature is around 80xc2x0 C.
These objects are accomplished by the characterising features of claim 1. Advantageous embodiments can be found in the dependant claims.
The new pigments show at least absorption within the blue-green spectral region. Furthermore they show fluorescent emission under absorption. Those Eu2+-doped luminescent materials show emission within the yellow to red spectral region, especially long wavelength red, orange or yellow emission. These pigments are based on alkaline-earth silicon nitride material as hostlattices. They are very promising, especially for LED applications, when used as phosphors. Hitherto white LEDs were realised by combining a blue emitting diode with a yellow emitting phosphor. Such a combination has only a poor colour rendition. A far better performance can be achieved by using a multicolour (for example red-green-blue) system. Typically the new material can be used together with a green-emitting (or yellow-emitting) phosphor, for example strontiumaluminate SrAl2O4:Eu2+, whose emission maximum is around 520 nm.
In detail, the new Pigment with day-light fluorescence uses a host lattice of the nitridosilicate type MxSiyNz:Eu, wherein M is at least one of an alkaline earth metal chosen from the group Ca, Sr, Ba and wherein z=⅔x+{fraction (4/3)}y. The incorporation of nitrogen increases the proportion of covalent bond and ligand-field splitting. As a consequence this leads to a pronounced shift of excitation and emission bands to longer wavelengths in comparison to oxide lattices.
Preferably, the pigment is of the type, wherein x=2, and y=5. In another preferred embodiment, the pigment is of the type, wherein x=1, and y=7.
Preferably, the metal M in the pigment is strontium because the resulting phosphor is emitting at relatively short yellow to red wavelengths. Thus the efficiency is rather high in comparison to most of the other elected metals M.
In a further embodiment the pigment uses a mixture of different metals, for example Ca (10 atom.-%) together with Ba (balance), as component M.
These materials show high absorption and good excitation in the UV and blue visible spectrum (up to more than 450 nm), high quantum efficiency and low temperature quenching up to 100xc2x0 C.
It can be used as a pigment for coloring goods or as a phosphor for luminescence conversion LEDs, especially with a blue light emitting primary source together with one or more other phosphors (red and green).